Problems with Apartment Management

I have seen a few posts here regarding issues with landlords and figured it'd be a good place to ask for help.

Now the issues are not mine. Someone I know (SWIM) is having these issues with their apartment managers/management team. All the info is based on what I've been told.

Since September, they have been receiving letters every month stating that rent is owed and that they need to pay the amount plus late fees or risk being locked out. Again, every month since September. Every month, they go in to dispute this issue with the management and every month they say it was an error and that the rent has been paid on time/nothing is owed. SWIM has kept money order receipts of all the payments that have been made and is the reason they are able to dispute these letters. The management team always blames someone else, usually the old management team, stating that "you aren't the only one having issues with this". I believe that the apartment complex has had a management change 3 times since September.

Now, today, they have received a letter stating they owe twice the previous amount stated plus late fees. Again, SWIM has the money orders to prove them wrong.

The issue here is that it's a recurring thing that keeps happening. All the letters state that the money owed is from the same month. SWIM has been recording the conversations (both in person and over the phone) where they state that it's a mistake (among other things).

Another issue is a mold problem that the management states "is because of the pool" but has yet to be fixed. This has been brought up to their attention since before the letters started.

Is there something that SWIM can do here legally in order to fix this? At this point, they feel they are being harassed and the constant threats of being locked out are stressing them out. Plus having to take the time to go into the office to dispute this every month.